!function(win, option) {
  var count = 0,
      designWidth = option.designWidth,
      designHeight = option.designHeight || 0,
      designFontSize = option.designFontSize || 20,
      callback = option.callback || null,
      root = document.documentElement,
      body = document.body,
      rootWidth, newSize, t, self;
      root.style.width = '100%';
  //返回root元素字体计算结果
  function _getNewFontSize() {
    var scale = designHeight !== 0 ? Math.min(win.innerWidth / designWidth, win.innerHeight / designHeight) : win.innerWidth / designWidth;
    return parseInt( scale * 10000 * designFontSize ) / 10000;
  }
  !function () {
    rootWidth = root.getBoundingClientRect().width;
    self = self ? self : arguments.callee;
    //如果此时屏幕宽度不准确，就尝试再次获取分辨率，只尝试20次，否则使用win.innerWidth计算
    if( rootWidth !== win.innerWidth &&  count < 20 ) {
      win.setTimeout(function () {
        count++;
        self();
      }, 0);
    } else {
      newSize = _getNewFontSize();
      //如果css已经兼容当前分辨率就不管了
      if( newSize + 'px' !== getComputedStyle(root)['font-size'] ) {
        root.style.fontSize = newSize + "px";
        return callback && callback(newSize);
      };
    };
  }();
  //横竖屏切换的时候改变fontSize，根据需要选择使用
  win.addEventListener("onorientationchange" in window ? "orientationchange" : "resize", function() {
    clearTimeout(t);
    t = setTimeout(function () {
      self();
    }, 300);
  }, false);
}(window, {
  designWidth: 640,
  designFontSize: 100,
  callback: function (argument) {

  }
});